here is a quick tutorial with crystaltile2.
Download crystaltile2 from this site. Click on the search button on the navigation bar, and type in crystaltile2, look for a post where FAST uploaded it, and download it. It doesn't require installing.
Once done, open up the crystaltile2 folder, and click on cystaltile2.exe, which should have an icon with a pink "CT".
click on the icon that looks like an opened folder (the "Open" button)
and select your japanese ROM. I don't want to waste another 20 minutes on downloading an english and a japanese ROM, so I'll just use Kimino Yusha (J) as an example.
Double click the Japanese version of whatever you want to undub, and you will see a hexeditor or a tile editor. Don't worry, and don't mess around with those.
on the below pic, I've put a RED box around what you need to click.
once you click that icon (looks like a DS), You'll get the list of files that are also in that screen.
Scroll down until you see a file called sound_data.sdat
It's a common file in many ROMs, but I can't promise that this file is in EVERY single NDS rom.
as you see, there will be a little icon with a speaker looking thingy on the left of the file, which also shows you that it is a sound file.
Now, once you've located your sound file, right click, and click on the first option. (also shown in the pic)
The first option should have an "(E)" on the end. Click on that option, and it will ask you where you want to save that file.
Just save it anywhere you wish.
Once you are done with that, repeat step 1 by opening the ROM, but this time, open the ENGLISH ROM.
Click on the DS icon looking button to bring the packed file list up for the English ROM.
Look for the sound data file just like step 2.
Once you've located your sound data file, right click.
This time, select the 2nd option, which should have an "(I)" at the end, just like this pic:
Now, it will ask you to select the file to open. Open the sound data file you've extracted from step 2 from the Japanese ROM, and press OK.
that's it.
Press save (should look like a floppy disk icon just in case you don't know; it's next to the Open button)
and now, go play your english rom. It should be undubbed.
